Abstract
Males and females of a species may differ in external appearance or other features. Sexual dimorphism often relates to mating behavior, via male-male competition for access to females (through direct fighting and/or indirect display), female choice of mates, or sexual conflict. In many mammals, skulls and teeth often display sexual dimorphism. Cetaceans show extraordinary variation in their dentition, and because teeth are often preserved, the evolutionary origins of these morphological novelties can be tracked in the fossil record. Sexual dimorphism has been proposed in several fossil cetaceans (i.e., pakicetids, protocetids, fossil beaked whales) and some odontocetes (notably Odobenocetops), and mainly inferred from differences in tooth size, skull dimensions, and thickening of skull bones. Within modern taxa, unusual differences in the dentition between the sexes have been observed in deep-diving beaked whales and arctic narwhals, the unicorns of the sea. Mandibular tusks in beaked whales are unusual because they erupt only in males, erupt only at sexual maturity, and protrude outside the mouth rather than projecting into the oral cavity. In beaked whales, the tusk-like dentition seems to have a minimal role in feeding, and functions as weapons or displays for intrasexual (e.g., male-male combat) and intersexual (e.g., female mate choice) competition, and possibly for female harassment by males seeking to mate. In narwhals, the long and spiralled left tusk commonly only erupts in males and is presumed to play a prominent role in male-male fighting or displays for female mate choice. Except for narwhals and beaked whales, sexual dimorphism in skull and dental structures is not prominent in cetaceans. However, we still do not know whether functional aspects such as enamel structure and thickness, mechanical properties, and chemical composition of dental tissues may vary between males and females.